Synthesis and Characterization of Photosensitive Epoxy Resins Containing Benzylidene and Cycloalkanone Moieties in the Main Chain

Abstract

Epoxy resins have been used successfully in a wide range ofapplications spanning from electronic products to advanced aerospacestructures. Photoreactive epoxy materials have recently gained a remarkableinterest, since the photochemical reactions in organic materials can inducemany changes in physico-chemical properties. Epoxy resins are widely usedin industries as protective coatings. The ability of the epoxy resin to react witha variety of substrates gives versatility to the epoxy resins.In the present study, photosensitive aromatic diol monomers wereinitially prepared. The aromatic diols, 2,6-bis(4-hydroxy-3-methoxybenzylidene) cyclohexanone (BHMBCH) and 2,5-bis(4-hydroxy-3-methoxybenzylidene) cyclopentanone (BHMBCP) were synthesized byreacting 4-hydroxy-3-methoxybenzaldehyde with cyclohexanone andcyclopentanone respectively in presence of borontrifluoride diethyletherate asa catalyst in ethanol medium. The monomers 2,6-bis(4-hydroxybenzylidene)cyclohexanone (BHBCH) and 2,5-bis(4-hydroxybenzylidene)cyclopentanone(BHBCP) were then synthesized by refluxing 4-hydroxybenzaldehyde withcyclohexanone and cyclopentanone respectively in ethanol medium and in thepresence of the borontrifluoride diethyletherate as catalyst.The epoxy resins were synthesized by a solution of aromatic diolswith epichlorohydrin in presence of base catalyst. The effect of the diolmonomer and epichlorohydrin ratio plays a major role in the properties ofepoxy resins. To study this effect all the epoxy resins were synthesized by ivvarying epichlorohydrin-diol monomer ratio. Bisphenol-A based epoxy resinwas synthesized by adopting the above said method to compare the propertieswith BHMBCH / BHMBCP / BHBCH / BHBCP based epoxy resins. Thecopolymers of bisphenol-A and BHMBCH / BHMBCP were also synthesizedby adopting the same procedure. The reaction conditions were optimized tosynthesize the epoxy resins.All the monomers and epoxy resins were synthesized in good yieldand the molecular structure was confirmed by elemental analysis, UV-Visible,FTIR,1H and13C-NMR spectroscopic techniques.The epoxy equivalent, determined using pyridinium hydrochloridemethod, showed that the epoxy equivalent decreases with increase inepichlorohydrin ratio. This may be due to the formation of diglycidylstructure at higher ratio of epichlorohydrin. The gel permeationchromatography results revealed that the synthesized epoxy resins are of lowmolecular weight.The synthesized oligomers were subjected to thermogravimetricanalysis and found that the oligomers containing cyclopentanone moietyexhibited higher thermal stability than the cyclohexanone containingoligomers. This may be due to the fact that the heat of combustion permethylene unit of cyclopentanone is higher than that of cyclohexanone. Thethermal stability of methoxy substituted epoxy resins was found to be morethan the unsubstituted counterparts. This may be due to the excess energyneeded to break the methoxy pendant group. vDifferential scanning calorimetric analysis results showed that thecyclopentanone containing epoxy oligomers possess higher meltingtemperature than the cyclohexanone containing oligomers. This may be due tohigher degree of crystallinity in cyclopentanone containing epoxy resins,which is in good agreement with X-ray diffraction studies. Further, it was alsofound that the transition temperature was inversely proportional to the size ofthe cycloalkanone ring. As the ring size increases, the hard segments startlosing their linear rigid structure, which reduced the transition temperature ofthe epoxy resins.The synthesized epoxy resins showed absorption maximum ataround 350nm due to ʌĺ ʌ* transition of the olefinic double bond present inthe main chain of oligomers. A decrease in the intensity of absorbance wasobserved in all oligomers during the successive irradiation with regular timeintervals. This may be attributed to the disappearance of double bond leadingto the formation of a cyclobutane ring by the 2ʌ + 2ʌ cycloaddition reactionof the olefinic double bond. Further, it was noticed that the rate ofphotocrosslinking of epoxy resins in solution state was higher than that in thefilm state. This could be due to the ease of orientation of chromophoricolefinic groups in solution phase for photocrosslinking.The photoreactive nature of the epoxy group present in the mainchain of the oligomers was studied by making a thin film from epoxy resinwith photoacid generator (PAG) and subjected to UV irradiation. The FTIRand DSC studies revealed that the epoxy resin with photoacid generator canundergo photopolymerization. These results substantiated that the synthesized viepoxy resins can undergo both photocrosslinking reaction between the–C=C- olefinic bonds and photopolymerization by epoxy rings. This studyconfirmed the presence of dual functionality in the synthesized epoxy resins.The thermal degradation kinetics of synthesized epoxy resins wasalso carried out using thermogravimetric analysis. The order of thermalstability was UV unexposed > UV exposed without PAG > UV exposed withPAG.The corrosion inhibition property of the synthesized epoxy resinwas studied over aluminium substrates in 3.5% NaCl solution. Thesynthesized epoxy resin was mixed with DDM, coated over the substrate andcured. Corrosion inhibition efficiency was measured by electrochemicalimpedance spectroscopy (EIS). The SEM images of the substrates were takenafter the corrosion experiment. The SEM and EIS results indicated that theepoxy resin forms a passive oxide film on the aluminium substrate and acts ascorrosion resistance layer.The results of this investigation indicate that synthesized epoxyresins can be used in photolithographic and coating applications.